Add support for structured validation report

[stefanprodan]

The flux-schema validate command can now emit a structured report of the validation results by setting --output to json or yaml. The envelope shape is versioned and documented by the JSON Schema.

Envelope

Every report is wrapped in a top-level envelope:

Key	Description
version	Wire format version. Currently "1.0.0".
$schema	URL of the JSON Schema describing the envelope.
report.reporter	Identity of the producer, e.g. flux-schema/0.1.0.
report.timestamp	RFC 3339 UTC timestamp of the run.
report.summary	Aggregate counts: total, valid, invalid, skipped.
report.results[]	One entry per validated document, in source order.

Result fields

Key	Description
resource	{apiVersion, kind, namespace?, name?} or null when no Kubernetes identity could be recovered (e.g. a file that fails to open, or stdin that is not YAML).
source	File path or (stdin).
idx	1-based position of the document within its source. 0 for source-level failures that have no document.
status	"valid", "invalid", or "skipped".
reason	Stable kebab-case code (see below). Omitted when status is "valid".
violations[]	Zero or more {path?, message} entries. path is a JSON Pointer set by schema violations; for every other reason it is absent and message carries the raw error text.

Reason enum

Reason	Triggered by	Status (default / with --skip-missing-schemas)
source-load-error	Source-level open/read failure.	invalid / invalid
yaml-parse-error	Strict YAML decode fails (duplicate keys, malformed doc).	invalid / invalid
schema-load-error	Schema loader failure (HTTP fetch, file read, or JSON Schema compile).	invalid / invalid
schema-not-found	No schema applicable — either no schema file matches the GVK, or the document has no GVK to look up.	invalid / skipped
schema-violation	Document fails one or more schema constraints. violations[] carries a JSON Pointer path per entry.	invalid / invalid
kind-skipped	Matched a --skip-kind pattern.	skipped / skipped

[matheuscscp]

LGTM! 🚀